Deglaciation and Postglacial Timberline in the San Juan Mountains, Colorado

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Abstract
Lake Emma, which no longer exists because of a mining accident, was a tarn in a south-facing cirque near the headwaters of the Animas River in the San Juan Mountains of southwestern Colorado. During the Pinedale glaciation, this area was covered by a large transection glacier centered over the Lake Emma region. Three radiocarbon dates on basal organic sediment from Lake Emma indicate that by ca. 15,000 yr B.P. this glacier, one of the largest in the southern Rocky Mountains, no longer existed. Twenty-two radiocarbon dates onPiceaandAbieskrummholz fragments in the Lake Emma deposits indicate that from ca. 9600 to 7800 yr B.P., from 6700 to 5600 yr B.P., and at 3100 yr B.P. the krummholz limit was at least 70 m higher than present. These data, in conjunction withPicea:Pinuspollen ratios from both the Lake Emma site and the Hurricane Basin site of J. T. Andrews, P. E. Carrara, F. B. King, and R. Struckenrath (1975,Quaternary Research5, 173–197) suggest than from ca. 9600 to 3000 yr B.P. timberline in the San Juan Mountains was higher than present. Cooling apparently began ca. 3000 yr B.P. as indicated by decreases in both the percentage ofPiceapollen andPicea:Pinuspollen ratios at the Hurricane Basin site (Andrewset al., 1975). Cooling is also suggested by the lack ofPiceaorAbiesfragments younger than 3000 yr B.P. at either the Lake Emma or the Hurricane Basin site.